The neurotransmitter serotonin influences several behaviors in animals (aggression, feeding, social behavior, sexual function) and is known to be abnormal in a growing number of human neurobehavioral conditions (anxiety, depression, autism, obsessions/compulsions). In addition, recently developed pharmacological agents that are dramatically effective in the treatment of anxiety, depression, and obsessions/compulsions, interact with specific serotonin receptor subtypes or uptake sites, suggesting that alterations in serotonin transmission are involved in the pharmacologic treatment of these disorders. Together, these findings suggest that altered serotonergic transmission is likely to be involved in the etiology and/or treatment of some of the most problematic behavioral disorders of children and adults with developmental disabilities and psychiatric disorders. Understanding the effects of early insults to the serotonin system on subsequent behavioral and neurobiologic function is critical for the understanding of the mechanism of development, prevention, and treatment of behavioral disorders involving serotonergic dysfunction. The serotonin-specific neurotoxin, 5,7-dihydroxytryptamine, (5, 7-DHT), can be used to induce lesions of serotonin terminals and neurons in neonatal rats. In this proposal, I plan to first characterize the behavioral, neurochemical, and histological patterns of deficits and recovery seen in these rats over the course of development. To test the hypothesis that alterations/ adaptations in the regulation of serotonin release may underlie functional deficit/ recovery after early injury, in vivo microdialysis will be used to measure serotonin release in response to a variety of challenges.